ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

A3.4.4 Context switch support
Application Level Memory Model
After a context switch, software must ensure that the local monitor is in the Open Access state. This requires
it to either:
execute a CLREX instruction
execute a dummy STREX to a memory address allocated for this purpose.
Note
Using a dummy STREX for this purpose is backwards-compatible with the ARMv6 implementation of
the exclusive operations. The CLREX instruction is introduced in ARMv6K.
Context switching is not an application level operation. However, this information is included here to
complete the description of the exclusive operations.
The STREX or CLREX instruction following a context switch might cause a subsequent Store-Exclusive to fail,
requiring a load … store sequence to be replayed. To minimize the possibility of this happening, ARM
recommends that the Store-Exclusive instruction is kept as close as possible to the associated
Load-Exclusive instruction, see Load-Exclusive and Store-Exclusive usage restrictions.
A3.4.5 Load-Exclusive and Store-Exclusive usage restrictions
The Load-Exclusive and Store-Exclusive instructions are intended to work together, as a pair, for example
a LDREX/STREX pair or a LDREXB/STREXB pair. As mentioned in Context switch support, ARM recommends that
the Store-Exclusive instruction always follows within a few instructions of its associated Load-Exclusive
instructions. To support different implementations of these functions, software must follow the notes and
restrictions given here.
These notes describe use of an LDREX/STREX pair, but apply equally to any other
Load-Exclusive/Store-Exclusive pair:
The exclusives support a single outstanding exclusive access for each processor thread that is
executed. The architecture makes use of this by not requiring an address or size check as part of the
IsExclusiveLocal() function. If the target address of an STREX is different from the preceding LDREX in
the same execution thread, behavior can be UNPREDICTABLE. As a result, an LDREX/STREX pair can only
be relied upon to eventually succeed if they are executed with the same address. Where a context
switch or exception might result in a change of execution thread, a CLREX instruction or a dummy
STREX instruction must be executed to avoid unwanted effects, as described in Context switch support
Using an STREX in this way is the only occasion where software can program an STREX with a different
address from the previously executed LDREX.
An explicit store to memory can cause the clearing of exclusive monitors associated with other
processors, therefore, performing a store between the LDREX and the STREX can result in a livelock
situation. As a result, code must avoid placing an explicit store between an LDREX and an STREX in a
single code sequence.
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