Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 9ad3c143 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

doc: De-emphasize smp_read_barrier_depends 



This commit keeps only the historical and low-level discussion of
smp_read_barrier_depends().

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Adjusted to allow for David Howells feedback on prior commit. ]
parent 98c1ec7c

Documentation/RCU/Design/Requirements/Requirements.html

+2 −1
Original line number Diff line number Diff line
@@ -581,7 +581,8 @@ This guarantee was only partially premeditated. 
DYNIX/ptx used an explicit memory barrier for publication, but had nothing

resembling <tt>rcu_dereference()</tt> for subscription, nor did it

have anything resembling the <tt>smp_read_barrier_depends()</tt>

that was later subsumed into <tt>rcu_dereference()</tt>.

that was later subsumed into <tt>rcu_dereference()</tt> and later

still into <tt>READ_ONCE()</tt>.

The need for these operations made itself known quite suddenly at a

late-1990s meeting with the DEC Alpha architects, back in the days when

DEC was still a free-standing company.

Documentation/RCU/rcu_dereference.txt

+1 −5
Original line number Diff line number Diff line
@@ -122,11 +122,7 @@ o Be very careful about comparing pointers obtained from 
		Note that if checks for being within an RCU read-side

		critical section are not required and the pointer is never

		dereferenced, rcu_access_pointer() should be used in place

		of rcu_dereference(). The rcu_access_pointer() primitive

		does not require an enclosing read-side critical section,

		and also omits the smp_read_barrier_depends() included in

		rcu_dereference(), which in turn should provide a small

		performance gain in some CPUs (e.g., the DEC Alpha).

		of rcu_dereference().



	o	The comparison is against a pointer that references memory

		that was initialized "a long time ago."  The reason

Documentation/RCU/whatisRCU.txt

+1 −2
Original line number Diff line number Diff line
@@ -600,8 +600,7 @@ don't forget about them when submitting patches making use of RCU!] 


	#define rcu_dereference(p) \

	({ \

		typeof(p) _________p1 = p; \

		smp_read_barrier_depends(); \

		typeof(p) _________p1 = READ_ONCE(p); \

		(_________p1); \

	})

Documentation/circular-buffers.txt

+1 −2
Original line number Diff line number Diff line
@@ -220,8 +220,7 @@ before it writes the new tail pointer, which will erase the item. 


Note the use of READ_ONCE() and smp_load_acquire() to read the

opposition index.  This prevents the compiler from discarding and

reloading its cached value - which some compilers will do across

smp_read_barrier_depends().  This isn't strictly needed if you can

reloading its cached value.  This isn't strictly needed if you can

be sure that the opposition index will _only_ be used the once.

The smp_load_acquire() additionally forces the CPU to order against

subsequent memory references.  Similarly, smp_store_release() is used

Documentation/memory-barriers.txt

+5 −2
Original line number Diff line number Diff line
@@ -1818,7 +1818,7 @@ The Linux kernel has eight basic CPU memory barriers: 
	GENERAL		mb()			smp_mb()

	WRITE		wmb()			smp_wmb()

	READ		rmb()			smp_rmb()

	DATA DEPENDENCY	read_barrier_depends()	smp_read_barrier_depends()

	DATA DEPENDENCY				READ_ONCE()





All memory barriers except the data dependency barriers imply a compiler
@@ -2867,7 +2867,10 @@ access depends on a read, not all do, so it may not be relied on. 


Other CPUs may also have split caches, but must coordinate between the various

cachelets for normal memory accesses.  The semantics of the Alpha removes the

need for coordination in the absence of memory barriers.

need for hardware coordination in the absence of memory barriers, which

permitted Alpha to sport higher CPU clock rates back in the day.  However,

please note that smp_read_barrier_depends() should not be used except in

Alpha arch-specific code and within the READ_ONCE() macro.





CACHE COHERENCY VS DMA