linux-kernel - Re: [patch] eventfd - remove fput() call from possible IRQ context (2nd rev)

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Message-ID: <49C116BF.9080402@cosmosbay.com>
Date:	Wed, 18 Mar 2009 16:43:59 +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 :
> Eric Dumazet <dada1@...mosbay.com> writes:
> 
> 
>>> 	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'm not sure why you think io_submit is blocking.  In my setup, I
> preallocated the file, and the test code opens it with O_DIRECT.  So,
> io_submit should return after the dio is issued, and the I/O size is
> large enough that it should still be outstanding when io_submit returns.

Hmm.. io_submit() is a blocking syscall, this is how I understood fs/aio.c

Then, using strace -tt -T on your program, I can confirm it is quite a long syscall (3.5 seconds,
about time needed to write a 256 MB file on my disk ;) )

16:33:15.861301 eventfd(0)              = 3 <0.000010>
16:33:15.861344 open("rwfile", O_RDWR|O_DIRECT) = 4 <0.000010>
16:33:15.861444 mmap(NULL, 268443648, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f08eec55000 <0.000008>
16:33:16.100040 io_setup(1024, {139676618563584}) = 0 <0.000047>
16:33:16.100149 io_submit(139676618563584, 1, {...}) = 1 <3.558085>
16:33:19.658292 io_getevents(139676618563584, 1, 1, {...}NULL) = 1 <0.258104>
16:33:19.916486 fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 0), ...}) = 0 <0.000007>
16:33:19.916559 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f08ff3b9000 <0.000009>
16:33:19.916607 write(1, "eventfd write test [SUCCESS]\n"..., 29eventfd write test [SUCCESS]
) = 29 <0.000008>
16:33:19.916653 exit_group(0)           = ?


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