Date: Thu, 27 Oct 2005 09:38:33 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: scottl@samsco.org Cc: freebsd-hackers@freebsd.org Subject: Re: locking in a device driver Message-ID: <20051027.093833.114360961.imp@bsdimp.com> In-Reply-To: <4360DD7B.20900@samsco.org> References: <200510261324.03790.jhb@freebsd.org> <4360B8EE.4070605@alphaque.com> <4360DD7B.20900@samsco.org>
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In message: <4360DD7B.20900@samsco.org>
Scott Long <scottl@samsco.org> writes:
: Dinesh Nair wrote:
: >
: > carrying on this discussion, what would be a good locking mechanism to
: > use to protect tsleep() and other sensitive areas in a driver in freebsd
: > 4.x ?
: >
: > the current code for the driver in 5.x uses mtx_lock and mtx_unlock with
: > some parts even being protected by mtx_lock(&Giant).
: >
: > would the use of simple_lock() or s_lock() do, given that
: > SIMPLELOCK_DEBUG was defined in the 4.x kernel ?
: >
: > the mechanism is actually a pseudo device driver which communicates with
: > the real device driver. the pseudo device driver creates a bunch of
: > /dev/ devices which the userland reads/writes to, and the pseudo device
: > driver then places data in a few buffers.
: >
: > the real device driver then reads these buffers and uses busdma to send
: > the data to the device. reading is done by using busdma to read from the
: > device and then placing the data in these buffers for the pseudo device
: > to return to the userland process.
: >
: > locking in the real device driver uses splhigh/splx, but what locking
: > should be used in the pseudo device driver ?
: >
:
: If you need to protect your pseudodriver from being interrupted by the
: real driver then you'll need to use the same spl() as the driver. Note
: that you shouldn't be using splhigh() unless you really know what you
: are doing. Other than that, there likely isn't anything that you need
: to do for 'locking' in 4.x. The kernel is non-reentrant there, so you
: don't need to worry about synchronizing multiple threads.
One thing to also bear in mind is that in 4.x spl locking is a code
lock. It keeps multiple 'threads' of execution from entering a block
of code. mutexes in -current are data locks. While usually one can
think of the two the same, it can trip the unweary up.
Locking in 4.x is indeed much simpler.
Warner
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