From: Paul E. McKenney Date: Wed, 10 Aug 2016 04:15:15 +0000 (-0700) Subject: documentation: Record reason for rcu_head two-byte alignment X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=ed2bec07fd1aa47f1c06be92c164c13c70fb7a45;p=linux-beck.git documentation: Record reason for rcu_head two-byte alignment There is an assertion in __call_rcu() that checks only the bottom bit of the rcu_head pointer, rather than the bottom two (as might be expected for 32-bit systems) or the bottom three (as might be expected for 64-bit systems). This choice might be a bit surprising in these days of ubiquitous 32-bit and 64-bit systems. This commit therefore records the reason for this odd alignment check, namely that m68k guarantees only two-byte alignment despite being a 32-bit architectures. Signed-off-by: Paul E. McKenney --- diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index ece410f40436..a4d3838130e4 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2493,6 +2493,28 @@ or some future “lazy” variant of call_rcu() that might one day be created for energy-efficiency purposes. +

+That said, there are limits. +RCU requires that the rcu_head structure be aligned to a +two-byte boundary, and passing a misaligned rcu_head +structure to one of the call_rcu() family of functions +will result in a splat. +It is therefore necessary to exercise caution when packing +structures containing fields of type rcu_head. +Why not a four-byte or even eight-byte alignment requirement? +Because the m68k architecture provides only two-byte alignment, +and thus acts as alignment's least common denominator. + +

+The reason for reserving the bottom bit of pointers to +rcu_head structures is to leave the door open to +“lazy” callbacks whose invocations can safely be deferred. +Deferring invocation could potentially have energy-efficiency +benefits, but only if the rate of non-lazy callbacks decreases +significantly for some important workload. +In the meantime, reserving the bottom bit keeps this option open +in case it one day becomes useful. +

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