diff options
Diffstat (limited to 'Documentation/core-api/this_cpu_ops.rst')
| -rw-r--r-- | Documentation/core-api/this_cpu_ops.rst | 24 |
1 files changed, 16 insertions, 8 deletions
diff --git a/Documentation/core-api/this_cpu_ops.rst b/Documentation/core-api/this_cpu_ops.rst index 5cb8b883ae83..533ac5dd5750 100644 --- a/Documentation/core-api/this_cpu_ops.rst +++ b/Documentation/core-api/this_cpu_ops.rst @@ -53,7 +53,6 @@ preemption and interrupts:: this_cpu_add_return(pcp, val) this_cpu_xchg(pcp, nval) this_cpu_cmpxchg(pcp, oval, nval) - this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2) this_cpu_sub(pcp, val) this_cpu_inc(pcp) this_cpu_dec(pcp) @@ -139,12 +138,22 @@ get_cpu/put_cpu sequence requires. No processor number is available. Instead, the offset of the local per cpu area is simply added to the per cpu offset. -Note that this operation is usually used in a code segment when -preemption has been disabled. The pointer is then used to -access local per cpu data in a critical section. When preemption -is re-enabled this pointer is usually no longer useful since it may -no longer point to per cpu data of the current processor. - +Note that this operation can only be used in code segments where +smp_processor_id() may be used, for example, where preemption has been +disabled. The pointer is then used to access local per cpu data in a +critical section. When preemption is re-enabled this pointer is usually +no longer useful since it may no longer point to per cpu data of the +current processor. + +The special cases where it makes sense to obtain a per-CPU pointer in +preemptible code are addressed by raw_cpu_ptr(), but such use cases need +to handle cases where two different CPUs are accessing the same per cpu +variable, which might well be that of a third CPU. These use cases are +typically performance optimizations. For example, SRCU implements a pair +of counters as a pair of per-CPU variables, and rcu_read_lock_nmisafe() +uses raw_cpu_ptr() to get a pointer to some CPU's counter, and uses +atomic_inc_long() to handle migration between the raw_cpu_ptr() and +the atomic_inc_long(). Per cpu variables and offsets ----------------------------- @@ -242,7 +251,6 @@ safe:: __this_cpu_add_return(pcp, val) __this_cpu_xchg(pcp, nval) __this_cpu_cmpxchg(pcp, oval, nval) - __this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2) __this_cpu_sub(pcp, val) __this_cpu_inc(pcp) __this_cpu_dec(pcp) |