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Date: Wed, 18 Mar 2009 15:55:39 +0100
From: Eric Dumazet <dada1@...mosbay.com>
To: Jeff Moyer <jmoyer@...hat.com>
CC: Davide Libenzi <davidel@...ilserver.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Benjamin LaHaise <bcrl@...ck.org>,
Trond Myklebust <trond.myklebust@....uio.no>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-aio <linux-aio@...ck.org>, zach.brown@...cle.com
Subject: Re: [patch] eventfd - remove fput() call from possible IRQ context
(2nd rev)
Jeff Moyer a écrit :
> Davide Libenzi <davidel@...ilserver.org> writes:
>
>> The following patch remove a possible source of fput() call from inside
>> IRQ context. Myself, like Eric, wasn't able to reproduce an fput() call
>> from IRQ context, but conceptually the bug is there.
>
> I've attached a test program which can reproduce the fput call in
> interrupt context. It's a modified version of the eventfd test that
> Rusty wrote for the libaio test harness. I verified that fput was in
> fact being called in interrupt context by using systemtap to print out
> the "thead_indent" of fput calls, and observing a "swapper(0)" in the
> output. After applying your patch, I confirmed that __fput is no longer
> called from interrupt context. Strangely enough, I never did get any
> output from the might_sleep in __fput. I can't explain that.
>
> I have some minor comments inlined below.
>
>> This patch adds an optimization similar to the one we already do on
>> ->ki_filp, on ->ki_eventfd. Playing with ->f_count directly is not pretty
>> in general, but the alternative here would be to add a brand new delayed
>> fput() infrastructure, that I'm not sure is worth it.
>>
>> On Sun, 15 Mar 2009, Benjamin LaHaise wrote:
>>
>>> This looks reasonably sane, the only concern I have with it is that I think
>>> it logically makes more sense to use the same convention for fi_filp and
>>> ki_eventfd, as the different in IS_ERR vs checking for NULL is a bit
>>> confusing. Aside from that, it looks like it should fix the problem
>>> correctly.
>> Makes sense.
>>
>> Signed-off-by: Davide Libenzi <davidel@...ilserver.org>
>>
>>
>> - Davide
>>
>>
>> ---
>> fs/aio.c | 37 +++++++++++++++++++++++++++----------
>> 1 file changed, 27 insertions(+), 10 deletions(-)
>>
>> Index: linux-2.6.mod/fs/aio.c
>> ===================================================================
>> --- linux-2.6.mod.orig/fs/aio.c 2009-03-14 09:24:12.000000000 -0700
>> +++ linux-2.6.mod/fs/aio.c 2009-03-15 12:54:10.000000000 -0700
> ...
>> @@ -527,12 +528,14 @@ static void aio_fput_routine(struct work
>> */
>> static int __aio_put_req(struct kioctx *ctx, struct kiocb *req)
>> {
>> + int schedule_putreq = 0;
>> +
>> dprintk(KERN_DEBUG "aio_put(%p): f_count=%ld\n",
>> req, atomic_long_read(&req->ki_filp->f_count));
>>
>> assert_spin_locked(&ctx->ctx_lock);
>>
>> - req->ki_users --;
>> + req->ki_users--;
>> BUG_ON(req->ki_users < 0);
>> if (likely(req->ki_users))
>> return 0;
>> @@ -540,10 +543,23 @@ static int __aio_put_req(struct kioctx *
>> req->ki_cancel = NULL;
>> req->ki_retry = NULL;
>>
>> - /* Must be done under the lock to serialise against cancellation.
>> - * Call this aio_fput as it duplicates fput via the fput_work.
>> + /*
>> + * Try to optimize the aio and eventfd file* puts, by avoiding to
>> + * schedule work in case it is not __fput() time. In normal cases,
>> + * we wouldn not be holding the last reference to the file*, so
> ^^^^^^^^^^
> tyop
>
>> + * this function will be executed w/out any aio kthread wakeup.
>> */
>> - if (unlikely(atomic_long_dec_and_test(&req->ki_filp->f_count))) {
>> + if (unlikely(atomic_long_dec_and_test(&req->ki_filp->f_count)))
>> + schedule_putreq++;
>> + else
>> + req->ki_filp = NULL;
>> + if (unlikely(req->ki_eventfd != NULL)) {
>> + if (unlikely(atomic_long_dec_and_test(&req->ki_eventfd->f_count)))
>> + schedule_putreq++;
>> + else
>> + req->ki_eventfd = NULL;
>> + }
>
> I agree with Jamie that you should get rid of the unlikely.
>
> Thanks for taking care of this, Davide.
>
> Signed-off-by: Jeff Moyer <jmoyer@...hat.com>
>
> Cheers,
> Jeff
>
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <string.h>
> #include <sys/types.h>
> #include <errno.h>
> #include <assert.h>
> #include <sys/eventfd.h>
> #include <libaio.h>
>
> int
> main(int argc, char **argv)
> {
> #define SIZE (256*1024*1024)
> char *buf;
> struct io_event io_event;
> struct iocb iocb;
> struct iocb *iocbs[] = { &iocb };
> int rwfd, efd;
> int res;
> io_context_t io_ctx;
>
> efd = eventfd(0, 0);
> if (efd < 0) {
> perror("eventfd");
> exit(1);
> }
>
> rwfd = open("rwfile", O_RDWR|O_DIRECT); assert(rwfd != -1);
> if (posix_memalign((void **)&buf, getpagesize(), SIZE) < 0) {
> perror("posix_memalign");
> exit(1);
> }
> memset(buf, 0x42, SIZE);
>
> /* Write test. */
> res = io_queue_init(1024, &io_ctx); assert(res == 0);
> io_prep_pwrite(&iocb, rwfd, buf, SIZE, 0);
> io_set_eventfd(&iocb, efd);
> res = io_submit(io_ctx, 1, iocbs); assert(res == 1);
yes but io_submit() is blocking. so your close(efd) will come after the release in fs/aio.c
I suggest you start a thread just before io_submit() and give it this work :
void *thread_work(void *arg)
{
usleep(10000);
close(efd);
return (void *)0;
}
>
> /* Now close the eventfd so that AIO has the last reference */
> close(efd);
>
> /* Keep this process around so that the aio subsystem does not hold
> * the last reference on the rwfd, otherwise the really_put_req will
> * be called from process context */
> res = io_getevents(io_ctx, 1, 1, &io_event, NULL);
> if (res != 1) {
> if (res < 0) {
> errno = -res;
> perror("io_getevents");
> } else
> printf("io_getevents did not return 1 event after "
> "closing eventfd\n");
> exit(1);
> }
> assert(io_event.res == SIZE);
> printf("eventfd write test [SUCCESS]\n");
>
> return 0;
> }
> /*
> * Local variables:
> * c-basic-offset: 8
> * compile-command: "gcc -o eventfd-in-irq eventfd-in-irq.c -laio -g3"
> * End:
> */
>
>
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