spinlock impl

SEMANTICS:

spinlocks are

• raw_spinlock_t (ALL)
• bit spinlocks (ALL)
• spinlock_t (NON-PREEMPT)
• rwlock_t (NON-PREEMPT)

on PREEMPT_RT kernels

{spinlock_t, rwlock_t, local_lock} are converted to sleeping locks

Well, on PREEMPT_RT kernels the spinlock_t is actually not a spinning lock.

spinlock apis

• spin_lock_init
• suffix bh : disables software interrupts (bottomhalves)
• suffix irq / irqsave: disable interrupts on local processors, do or do not preserve the interrupt state in the flags (restore via _irqrestore)
• spin_lock and spin_unlock: acquire and release the lock

raw_spinlock_t

raw_spinlock_t is strict spinning lock for all kernels. On NON-PREEMPT kernels spinlock_t and raw_spinlock_t have same semantics.

// THREAD 1
spin_lock(l);
// critical section <- PREEMPTED
spin_unlock(l);

// THREAD 2
spin_lock(l);    // spinning forever
spin_unlock(l);

// KERNEL CODE
spin_lock(l);
// interrupted here ------>
read_and_work(buf);
spin_unlock(l);

// INTERRUPT HANDLER
spin_lock(l);   // spins forever
spin_unlock(l);

linux code:

• raw_spinlock_t is defined in spinlock_types_raw.h
• the underlying arch_spinlock_t could be implemented as either queued spinlock; ticket lock arm or a primitive int.

queued spinlock

/*
 * By using the whole 2nd least significant byte for the
 * pending bit, we can allow better optimization of the lock
 * acquisition for the pending bit holder.
 */

typedef struct qspinlock {
    union {
        atomic_t val;
        struct {
            u8  locked;
            u8  pending;
        };
        struct {
            u16 locked_pending;
            u16 tail;
        };
    };
} arch_spinlock_t;

SOURCE CODE FILES

include/linux/spinlock.h

generic spinlock/rwlock declarations

here’s the role of the various spinlock/rwlock related include files:

on SMP builds:

asm/spinlock_types.h

contains the arch_spinlock_t/arch_rwlock_t and the initializers

linux/spinlock_types_raw

The raw types and initializers

linux/spinlock_types.h

defines the generic type and initializers

asm/spinlock.h

contains the arch_spin_*()/etc. lowlevel implementations, mostly inline assembly code

(also included on UP-debug builds:)

linux/spinlock_api_smp.h

contains the prototypes for the spin*() APIs.

linux/spinlock.h

builds the final spin_*() APIs.

on UP builds:

linux/spinlock_type_up.h

contains the generic, simplified UP spinlock type. (which is an empty structure on non-debug builds)

linux/spinlock_types_raw

The raw RT types and initializers linux/spinlock_types.h:

defines the generic type and initializers linux/spinlock_up.h:

contains the arch_spin_*()/etc. version of UP builds. (which are NOPs on non-debug, non-preempt builds)

(included on UP-non-debug builds:)

linux/spinlock_api_up.h

builds the spin*() APIs.

linux/spinlock.h

builds the final spin_*() APIs.