[Mesa-dev] [PATCH v3] mesa: Add new fast mtx_t mutex type for basic use cases

[Nicolai Hähnle]

What's the status of this?

On 16.10.2017 14:36, Emil Velikov wrote:
> On 16 October 2017 at 08:06, Timothy Arceri <tarceri at itsqueeze.com> wrote:
>> While modern pthread mutexes are very fast, they still incur a call to an
>> external DSO and overhead of the generality and features of pthread mutexes.
>> Most mutexes in mesa only needs lock/unlock, and the idea here is that we can
>> inline the atomic operation and make the fast case just two intructions.
>> Mutexes are subtle and finicky to implement, so we carefully copy the
>> implementation from Ulrich Dreppers well-written and well-reviewed paper:
>>
>>    "Futexes Are Tricky"
>>    http://www.akkadia.org/drepper/futex.pdf
>>
>> We implement "mutex3", which gives us a mutex that has no syscalls on
>> uncontended lock or unlock.  Further, the uncontended case boils down to a
>> cmpxchg and an untaken branch and the uncontended unlock is just a locked decr
>> and an untaken branch.  We use __builtin_expect() to indicate that contention
>> is unlikely so that gcc will put the contention code out of the main code
>> flow.
>>
> Worth using likely/unlikely macros from src/util/macros.h?
> 
>>   configure.ac                     |   3 +
>>   src/intel/vulkan/anv_allocator.c |  24 +-----
>>   src/util/Makefile.sources        |   1 +
>>   src/util/meson.build             |   1 +
>>   src/util/simple_mtx.h            | 153 +++++++++++++++++++++++++++++++++++++++
>>   5 files changed, 159 insertions(+), 23 deletions(-)
>>   create mode 100644 src/util/simple_mtx.h
>>
>> diff --git a/configure.ac b/configure.ac
>> index 62d33a1941c..c113e714638 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -894,6 +894,9 @@ if test "x$pthread_stubs_possible" = xyes; then
>>       PKG_CHECK_MODULES(PTHREADSTUBS, pthread-stubs >= 0.4)
>>   fi
>>
>> +dnl Check for futex for fast inline simple_mtx_t.
>> +AC_CHECK_HEADER([linux/futex.h], [DEFINES="$DEFINES -DHAVE_FUTEX"])
>> +
> Dylan you might want a similar check for Meson?
> 
> 
>> +static inline int futex_wake(uint32_t *addr, int count) {
>> +   return sys_futex(addr, FUTEX_WAKE, count, NULL, NULL, 0);
>> +}
>> +
>> +static inline int futex_wait(uint32_t *addr, int32_t value) {
>> +   return sys_futex(addr, FUTEX_WAIT, value, NULL, NULL, 0);
>> +}
> Coding style: please keep { on the next line.
> 
> -Emil
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