在 2021/8/21 上午2:59, Pavel Begunkov 写道:
On 8/20/21 7:40 PM, Hao Xu wrote:
coml_nr in ctx_flush_and_put() is not protected by uring_lock, this
may cause problems when accessing it parallelly.
Did you hit any problem? It sounds like it should be fine as is:
The trick is that it's only responsible to flush requests added
during execution of current call to tctx_task_work(), and those
naturally synchronised with the current task. All other potentially
enqueued requests will be of someone else's responsibility.
So, if nobody flushed requests, we're finely in-sync. If we see
0 there, but actually enqueued a request, it means someone
actually flushed it after the request had been added.
Probably, needs a more formal explanation with happens-before
and so.
I should put more detail in the commit message, the thing is:
say coml_nr > 0
ctx_flush_and put other context
if (compl_nr) get mutex
coml_nr > 0
do flush
coml_nr = 0
release mutex
get mutex
do flush (*)
release mutex
in (*) place, we do a bunch of unnecessary works, moreover, we
call io_cqring_ev_posted() which I think we shouldn't.
Fixes: d10299e14aae ("io_uring: inline struct io_comp_state")
FWIW, it came much earlier than this commit, IIRC
commit 2c32395d8111037ae2cb8cab883e80bcdbb70713
Author: Pavel Begunkov <asml.silence@xxxxxxxxx>
Date: Sun Feb 28 22:04:53 2021 +0000
io_uring: fix __tctx_task_work() ctx race
Signed-off-by: Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>
---
fs/io_uring.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index c755efdac71f..420f8dfa5327 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2003,11 +2003,10 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx)
{
if (!ctx)
return;
- if (ctx->submit_state.compl_nr) {
- mutex_lock(&ctx->uring_lock);
+ mutex_lock(&ctx->uring_lock);
+ if (ctx->submit_state.compl_nr)
io_submit_flush_completions(ctx);
- mutex_unlock(&ctx->uring_lock);
- }
+ mutex_unlock(&ctx->uring_lock);
percpu_ref_put(&ctx->refs);
}
