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vfs_io_uring: avoid stack recursion of vfs_io_uring_queue_run()

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Instead we remember if recursion was triggered and jump to
the start of the function again from the end.

This should make it safe to be called from the completion_fn().

This is hideously complex stuff, so document the hell
out of it.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14361

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
This commit is contained in:
Stefan Metzmacher 2020-05-08 21:29:53 +02:00 committed by Jeremy Allison
parent 388bc2e6e4
commit f96f45c9ba
1 changed files with 92 additions and 1 deletions
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93
source3/modules/vfs_io_uring.c
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@ -34,6 +34,10 @@ struct vfs_io_uring_request;
struct vfs_io_uring_config {
struct io_uring uring;
struct tevent_fd *fde;
/* recursion guard. See comment above vfs_io_uring_queue_run() */
bool busy;
/* recursion guard. See comment above vfs_io_uring_queue_run() */
bool need_retry;
struct vfs_io_uring_request *queue;
struct vfs_io_uring_request *pending;
};
@ -222,7 +226,7 @@ static int vfs_io_uring_connect(vfs_handle_struct *handle, const char *service,
return 0;
}
static void vfs_io_uring_queue_run(struct vfs_io_uring_config *config)
static void _vfs_io_uring_queue_run(struct vfs_io_uring_config *config)
{
struct vfs_io_uring_request *cur = NULL, *next = NULL;
struct io_uring_cqe *cqe = NULL;
@ -280,6 +284,93 @@ static void vfs_io_uring_queue_run(struct vfs_io_uring_config *config)
io_uring_cq_advance(&config->uring, nr);
}
/*
* Wrapper function to prevent recursion which could happen
* if we called _vfs_io_uring_queue_run() directly without
* recursion checks.
*
* Looking at the pread call, we can have:
*
* vfs_io_uring_pread_send()
* ->vfs_io_uring_pread_submit() <-----------------------------------
* ->vfs_io_uring_request_submit() |
* ->vfs_io_uring_queue_run() |
* ->_vfs_io_uring_queue_run() |
* |
* But inside _vfs_io_uring_queue_run() looks like: |
* |
* _vfs_io_uring_queue_run() { |
* if (THIS_IO_COMPLETED) { |
* ->vfs_io_uring_finish_req() |
* ->cur->completion_fn() |
* } |
* } |
* |
* cur->completion_fn() for pread is set to vfs_io_uring_pread_completion() |
* |
* vfs_io_uring_pread_completion() { |
* if (READ_TERMINATED) { |
* -> tevent_req_done() - We're done, go back up the stack. |
* return; |
* } |
* |
* We have a short read - adjust the io vectors |
* |
* ->vfs_io_uring_pread_submit() ---------------------------------------
* }
*
* So before calling _vfs_io_uring_queue_run() we backet it with setting
* a flag config->busy, and unset it once _vfs_io_uring_queue_run() finally
* exits the retry loop.
*
* If we end up back into vfs_io_uring_queue_run() we notice we've done so
* as config->busy is set and don't recurse into _vfs_io_uring_queue_run().
*
* We set the second flag config->need_retry that tells us to loop in the
* vfs_io_uring_queue_run() call above us in the stack and return.
*
* When the outer call to _vfs_io_uring_queue_run() returns we are in
* a loop checking if config->need_retry was set. That happens if
* the short read case occurs and _vfs_io_uring_queue_run() ended up
* recursing into vfs_io_uring_queue_run().
*
* Once vfs_io_uring_pread_completion() finishes without a short
* read (the READ_TERMINATED case, tevent_req_done() is called)
* then config->need_retry is left as false, we exit the loop,
* set config->busy to false so the next top level call into
* vfs_io_uring_queue_run() won't think it's a recursed call
* and return.
*
*/
static void vfs_io_uring_queue_run(struct vfs_io_uring_config *config)
{
if (config->busy) {
/*
* We've recursed due to short read/write.
* Set need_retry to ensure we retry the
* io_uring_submit().
*/
config->need_retry = true;
return;
}
/*
* Bracket the loop calling _vfs_io_uring_queue_run()
* with busy = true / busy = false.
* so we can detect recursion above.
*/
config->busy = true;
do {
config->need_retry = false;
_vfs_io_uring_queue_run(config);
} while (config->need_retry);
config->busy = false;
}
static void vfs_io_uring_fd_handler(struct tevent_context *ev,
struct tevent_fd *fde,
uint16_t flags,