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userfaultfd: Use a seqlock instead of seqcount 

On RT write_seqcount_begin() disables preemption which leads to warning
in add_wait_queue() while the spinlock_t is acquired.
The waitqueue can't be converted to swait_queue because
userfaultfd_wake_function() is used as a custom wake function.

Use seqlock instead seqcount to avoid the preempt_disable() section
during add_wait_queue().

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
This commit is contained in:
Sebastian Andrzej Siewior 2019-12-18 12:25:09 +01:00 committed by Alibek Omarov
parent c587de4dc8
commit cac578e31b
1 changed files with 6 additions and 6 deletions
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12
fs/userfaultfd.c
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@ -56,7 +56,7 @@ struct userfaultfd_ctx {
/* waitqueue head for events */
wait_queue_head_t event_wqh;
/* a refile sequence protected by fault_pending_wqh lock */
struct seqcount refile_seq;
seqlock_t refile_seq;
/* pseudo fd refcounting */
refcount_t refcount;
/* userfaultfd syscall flags */
@ -1058,7 +1058,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
* waitqueue could become empty if this is the
* only userfault.
*/
write_seqcount_begin(&ctx->refile_seq);
write_seqlock(&ctx->refile_seq);
/*
* The fault_pending_wqh.lock prevents the uwq
@ -1084,7 +1084,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
list_del(&uwq->wq.entry);
add_wait_queue(&ctx->fault_wqh, &uwq->wq);
write_seqcount_end(&ctx->refile_seq);
write_sequnlock(&ctx->refile_seq);
/* careful to always initialize msg if ret == 0 */
*msg = uwq->msg;
@ -1257,11 +1257,11 @@ static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
* sure we've userfaults to wake.
*/
do {
seq = read_seqcount_begin(&ctx->refile_seq);
seq = read_seqbegin(&ctx->refile_seq);
need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
waitqueue_active(&ctx->fault_wqh);
cond_resched();
} while (read_seqcount_retry(&ctx->refile_seq, seq));
} while (read_seqretry(&ctx->refile_seq, seq));
if (need_wakeup)
__wake_userfault(ctx, range);
}
@ -1935,7 +1935,7 @@ static void init_once_userfaultfd_ctx(void *mem)
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->event_wqh);
init_waitqueue_head(&ctx->fd_wqh);
seqcount_init(&ctx->refile_seq);
seqlock_init(&ctx->refile_seq);
}
SYSCALL_DEFINE1(userfaultfd, int, flags)