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From: Kevin Wolf <kwolf@redhat.com>
To: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"berrange@redhat.com" <berrange@redhat.com>,
	Denis Lunev <den@virtuozzo.com>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"mreitz@redhat.com" <mreitz@redhat.com>,
	Roman Kagan <rkagan@virtuozzo.com>
Subject: Re: [Qemu-devel] [PATCH v2 7/7] iotests: new file to suppress Valgrind errors
Date: Tue, 25 Jun 2019 10:13:31 +0200	[thread overview]
Message-ID: <20190625081331.GB5187@localhost.localdomain> (raw)
In-Reply-To: <09632364-3d1d-9e5c-a050-f48f76d2e38c@virtuozzo.com>

Am 24.06.2019 um 18:55 hat Andrey Shinkevich geschrieben:
> 
> 
> On 17/06/2019 14:45, Kevin Wolf wrote:
> > Am 11.06.2019 um 20:02 hat Andrey Shinkevich geschrieben:
> >> The Valgrind tool reports about an uninitialised memory usage when the
> >> initialization is actually not needed. For example, the buffer 'buf'
> >> instantiated on a stack of the function guess_disk_lchs().
> > 
> > I would be careful with calling initialisation "not needed". It means
> > that the test case may not behave entirely determinstic because the
> > uninitialised memory can vary between runs.\
> 
> I am going to amend the comment.
> 
> Andrey
> 
> > 
> > In this specific case, I assume that guess_disk_lchs() is called for a
> > null block node, for which .bdrv_co_preadv by default returns without
> > actually writing to the buffer. Instead of ignoring the valgrind error,
> > we could instead pass read-zeroes=on to the null block driver to make
> > the test deterministic.
> 
> The buffer that the Valgrind complains of is initialized by the 
> following function call blk_pread_unthrottled() that reads the first 
> BDRV_SECTOR_SIZE bytes form a disk "to guess the disk logical geometry". 
> The Valgrind does not recognize that way of initialization. I believe we 
> do not need to zero the buffer instantiated on the stack just to make 
> the Valgrind silent there.

My point is that blk_pread_unthrottled() with null-co/null-aio leaves
the buffer untouched if read-zeroes=off (which is the default). So yes,
valgrind is right, this memory is still uninitialised after
blk_pread_unthrottled().

Kevin


      reply	other threads:[~2019-06-25  8:15 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-11 18:02 [Qemu-devel] [PATCH v2 0/7] Allow Valgrind checking all QEMU processes Andrey Shinkevich
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 1/7] iotests: allow " Andrey Shinkevich
2019-06-13  9:44   ` Vladimir Sementsov-Ogievskiy
2019-06-27 15:08     ` Andrey Shinkevich
2019-06-13  9:45   ` Vladimir Sementsov-Ogievskiy
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 2/7] iotests: exclude killed processes from running under Valgrind Andrey Shinkevich
2019-06-13  9:47   ` Vladimir Sementsov-Ogievskiy
2019-06-17 11:57     ` Roman Kagan
2019-06-17 11:15   ` Kevin Wolf
2019-06-17 12:18     ` Roman Kagan
2019-06-17 12:53       ` Kevin Wolf
2019-06-17 13:20         ` Roman Kagan
2019-06-17 14:51           ` Kevin Wolf
2019-06-24 16:55             ` Andrey Shinkevich
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 3/7] iotests: Valgrind fails to work with nonexistent directory Andrey Shinkevich
2019-06-13  9:52   ` Vladimir Sementsov-Ogievskiy
2019-06-17 11:22     ` Kevin Wolf
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 4/7] iotests: extended timeout under Valgrind Andrey Shinkevich
2019-06-13  9:54   ` Vladimir Sementsov-Ogievskiy
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 5/7] iotests: extend sleeping time " Andrey Shinkevich
2019-06-13  9:55   ` Vladimir Sementsov-Ogievskiy
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 6/7] iotests: amend QEMU NBD process synchronization Andrey Shinkevich
2019-06-13  9:59   ` Vladimir Sementsov-Ogievskiy
2019-06-17 12:45     ` Roman Kagan
2019-06-17 12:38   ` Roman Kagan
2019-06-11 18:02 ` [Qemu-devel] [PATCH v2 7/7] iotests: new file to suppress Valgrind errors Andrey Shinkevich
2019-06-13 10:06   ` Vladimir Sementsov-Ogievskiy
2019-06-24 16:55     ` Andrey Shinkevich
2019-06-24 17:09       ` Eric Blake
2019-06-24 17:23         ` Andrey Shinkevich
2019-06-17 11:45   ` Kevin Wolf
2019-06-24 16:55     ` Andrey Shinkevich
2019-06-25  8:13       ` Kevin Wolf [this message]

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