qemu-e2k/hw/9pfs/trace-events

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# See docs/devel/tracing.txt for syntax documentation.
# hw/9pfs/virtio-9p.c
9pfs: Correctly handle cancelled requests # Background I was investigating spurious non-deterministic EINTR returns from various 9p file system operations in a Linux guest served from the qemu 9p server. ## EINTR, ERESTARTSYS and the linux kernel When a signal arrives that the Linux kernel needs to deliver to user-space while a given thread is blocked (in the 9p case waiting for a reply to its request in 9p_client_rpc -> wait_event_interruptible), it asks whatever driver is currently running to abort its current operation (in the 9p case causing the submission of a TFLUSH message) and return to user space. In these situations, the error message reported is generally ERESTARTSYS. If the userspace processes specified SA_RESTART, this means that the system call will get restarted upon completion of the signal handler delivery (assuming the signal handler doesn't modify the process state in complicated ways not relevant here). If SA_RESTART is not specified, ERESTARTSYS gets translated to EINTR and user space is expected to handle the restart itself. ## The 9p TFLUSH command The 9p TFLUSH commands requests that the server abort an ongoing operation. The man page [1] specifies: ``` If it recognizes oldtag as the tag of a pending transaction, it should abort any pending response and discard that tag. [...] When the client sends a Tflush, it must wait to receive the corresponding Rflush before reusing oldtag for subsequent messages. If a response to the flushed request is received before the Rflush, the client must honor the response as if it had not been flushed, since the completed request may signify a state change in the server ``` In particular, this means that the server must not send a reply with the orignal tag in response to the cancellation request, because the client is obligated to interpret such a reply as a coincidental reply to the original request. # The bug When qemu receives a TFlush request, it sets the `cancelled` flag on the relevant pdu. This flag is periodically checked, e.g. in `v9fs_co_name_to_path`, and if set, the operation is aborted and the error is set to EINTR. However, the server then violates the spec, by returning to the client an Rerror response, rather than discarding the message entirely. As a result, the client is required to assume that said Rerror response is a result of the original request, not a result of the cancellation and thus passes the EINTR error back to user space. This is not the worst thing it could do, however as discussed above, the correct error code would have been ERESTARTSYS, such that user space programs with SA_RESTART set get correctly restarted upon completion of the signal handler. Instead, such programs get spurious EINTR results that they were not expecting to handle. It should be noted that there are plenty of user space programs that do not set SA_RESTART and do not correctly handle EINTR either. However, that is then a userspace bug. It should also be noted that this bug has been mitigated by a recent commit to the Linux kernel [2], which essentially prevents the kernel from sending Tflush requests unless the process is about to die (in which case the process likely doesn't care about the response). Nevertheless, for older kernels and to comply with the spec, I believe this change is beneficial. # Implementation The fix is fairly simple, just skipping notification of a reply if the pdu was previously cancelled. We do however, also notify the transport layer that we're doing this, so it can clean up any resources it may be holding. I also added a new trace event to distinguish operations that caused an error reply from those that were cancelled. One complication is that we only omit sending the message on EINTR errors in order to avoid confusing the rest of the code (which may assume that a client knows about a fid if it sucessfully passed it off to pud_complete without checking for cancellation status). This does mean that if the server acts upon the cancellation flag, it always needs to set err to EINTR. I believe this is true of the current code. [1] https://9fans.github.io/plan9port/man/man9/flush.html [2] https://github.com/torvalds/linux/commit/9523feac272ccad2ad8186ba4fcc891 Signed-off-by: Keno Fischer <keno@juliacomputing.com> Reviewed-by: Greg Kurz <groug@kaod.org> [groug, send a zero-sized reply instead of detaching the buffer] Signed-off-by: Greg Kurz <groug@kaod.org> Acked-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
2018-02-01 21:21:27 +01:00
v9fs_rcancel(uint16_t tag, uint8_t id) "tag %d id %d"
v9fs_rerror(uint16_t tag, uint8_t id, int err) "tag %d id %d err %d"
v9fs_version(uint16_t tag, uint8_t id, int32_t msize, char* version) "tag %d id %d msize %d version %s"
v9fs_version_return(uint16_t tag, uint8_t id, int32_t msize, char* version) "tag %d id %d msize %d version %s"
v9fs_attach(uint16_t tag, uint8_t id, int32_t fid, int32_t afid, char* uname, char* aname) "tag %u id %u fid %d afid %d uname %s aname %s"
v9fs_attach_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path) "tag %d id %d type %d version %d path %"PRId64
v9fs_stat(uint16_t tag, uint8_t id, int32_t fid) "tag %d id %d fid %d"
v9fs_stat_return(uint16_t tag, uint8_t id, int32_t mode, int32_t atime, int32_t mtime, int64_t length) "tag %d id %d stat={mode %d atime %d mtime %d length %"PRId64"}"
v9fs_getattr(uint16_t tag, uint8_t id, int32_t fid, uint64_t request_mask) "tag %d id %d fid %d request_mask %"PRIu64
v9fs_getattr_return(uint16_t tag, uint8_t id, uint64_t result_mask, uint32_t mode, uint32_t uid, uint32_t gid) "tag %d id %d getattr={result_mask %"PRId64" mode %u uid %u gid %u}"
v9fs_walk(uint16_t tag, uint8_t id, int32_t fid, int32_t newfid, uint16_t nwnames) "tag %d id %d fid %d newfid %d nwnames %d"
v9fs_walk_return(uint16_t tag, uint8_t id, uint16_t nwnames, void* qids) "tag %d id %d nwnames %d qids %p"
v9fs_open(uint16_t tag, uint8_t id, int32_t fid, int32_t mode) "tag %d id %d fid %d mode %d"
v9fs_open_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path, int iounit) "tag %d id %d qid={type %d version %d path %"PRId64"} iounit %d"
v9fs_lcreate(uint16_t tag, uint8_t id, int32_t dfid, int32_t flags, int32_t mode, uint32_t gid) "tag %d id %d dfid %d flags %d mode %d gid %u"
v9fs_lcreate_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path, int32_t iounit) "tag %d id %d qid={type %d version %d path %"PRId64"} iounit %d"
v9fs_fsync(uint16_t tag, uint8_t id, int32_t fid, int datasync) "tag %d id %d fid %d datasync %d"
v9fs_clunk(uint16_t tag, uint8_t id, int32_t fid) "tag %d id %d fid %d"
v9fs_read(uint16_t tag, uint8_t id, int32_t fid, uint64_t off, uint32_t max_count) "tag %d id %d fid %d off %"PRIu64" max_count %u"
v9fs_read_return(uint16_t tag, uint8_t id, int32_t count, ssize_t err) "tag %d id %d count %d err %zd"
v9fs_readdir(uint16_t tag, uint8_t id, int32_t fid, uint64_t offset, uint32_t max_count) "tag %d id %d fid %d offset %"PRIu64" max_count %u"
v9fs_readdir_return(uint16_t tag, uint8_t id, uint32_t count, ssize_t retval) "tag %d id %d count %u retval %zd"
v9fs_write(uint16_t tag, uint8_t id, int32_t fid, uint64_t off, uint32_t count, int cnt) "tag %d id %d fid %d off %"PRIu64" count %u cnt %d"
v9fs_write_return(uint16_t tag, uint8_t id, int32_t total, ssize_t err) "tag %d id %d total %d err %zd"
v9fs_create(uint16_t tag, uint8_t id, int32_t fid, char* name, int32_t perm, int8_t mode) "tag %d id %d fid %d name %s perm %d mode %d"
v9fs_create_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path, int iounit) "tag %d id %d qid={type %d version %d path %"PRId64"} iounit %d"
v9fs_symlink(uint16_t tag, uint8_t id, int32_t fid, char* name, char* symname, uint32_t gid) "tag %d id %d fid %d name %s symname %s gid %u"
v9fs_symlink_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path) "tag %d id %d qid={type %d version %d path %"PRId64"}"
v9fs_flush(uint16_t tag, uint8_t id, int16_t flush_tag) "tag %d id %d flush_tag %d"
v9fs_link(uint16_t tag, uint8_t id, int32_t dfid, int32_t oldfid, char* name) "tag %d id %d dfid %d oldfid %d name %s"
v9fs_remove(uint16_t tag, uint8_t id, int32_t fid) "tag %d id %d fid %d"
v9fs_wstat(uint16_t tag, uint8_t id, int32_t fid, int32_t mode, int32_t atime, int32_t mtime) "tag %u id %u fid %d stat={mode %d atime %d mtime %d}"
v9fs_mknod(uint16_t tag, uint8_t id, int32_t fid, int mode, int major, int minor) "tag %d id %d fid %d mode %d major %d minor %d"
v9fs_mknod_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path) "tag %d id %d qid={type %d version %d path %"PRId64"}"
v9fs_lock(uint16_t tag, uint8_t id, int32_t fid, uint8_t type, uint64_t start, uint64_t length) "tag %d id %d fid %d type %d start %"PRIu64" length %"PRIu64
v9fs_lock_return(uint16_t tag, uint8_t id, int8_t status) "tag %d id %d status %d"
v9fs_getlock(uint16_t tag, uint8_t id, int32_t fid, uint8_t type, uint64_t start, uint64_t length)"tag %d id %d fid %d type %d start %"PRIu64" length %"PRIu64
v9fs_getlock_return(uint16_t tag, uint8_t id, uint8_t type, uint64_t start, uint64_t length, uint32_t proc_id) "tag %d id %d type %d start %"PRIu64" length %"PRIu64" proc_id %u"
v9fs_mkdir(uint16_t tag, uint8_t id, int32_t fid, char* name, int mode, uint32_t gid) "tag %u id %u fid %d name %s mode %d gid %u"
v9fs_mkdir_return(uint16_t tag, uint8_t id, int8_t type, int32_t version, int64_t path, int err) "tag %u id %u qid={type %d version %d path %"PRId64"} err %d"
v9fs_xattrwalk(uint16_t tag, uint8_t id, int32_t fid, int32_t newfid, char* name) "tag %d id %d fid %d newfid %d name %s"
v9fs_xattrwalk_return(uint16_t tag, uint8_t id, int64_t size) "tag %d id %d size %"PRId64
v9fs_xattrcreate(uint16_t tag, uint8_t id, int32_t fid, char* name, uint64_t size, int flags) "tag %d id %d fid %d name %s size %"PRIu64" flags %d"
v9fs_readlink(uint16_t tag, uint8_t id, int32_t fid) "tag %d id %d fid %d"
v9fs_readlink_return(uint16_t tag, uint8_t id, char* target) "tag %d id %d name %s"