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Re: [dm-devel] [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex



On 11/7/06, Alasdair G Kergon <agk redhat com> wrote:
From: Srinivasa Ds <srinivasa in ibm com>

On debugging I found out that,"dmsetup suspend <device name>" calls
"freeze_bdev()",which locks "bd_mount_mutex" to make sure that no new mounts
happen on bdev until thaw_bdev() is called.  This "thaw_bdev()" is getting
called when we resume the device through "dmsetup resume <device-name>".
Hence we have 2 processes,one of which locks "bd_mount_mutex"(dmsetup
suspend) and another(dmsetup resume) unlocks it.

Srinivasa's description of the patch just speaks to how freeze_bdev
and thaw_bdev are used by DM but completely skips justification for
switching from mutex to semaphore.  Why is it beneficial and/or
necessary to use a semaphore instead of a mutex here?

Mike


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