Understanding the Linux kernel for interrupt saving and recovery

In the Linux kernel (linux-4.14.12/mm/slab.c#3389), at first glance at the bottom of the code, it seems that L3389 have a bug, so I read around the interest of a bit local_irq_save/local_irq_ The source code of the restore.

/*linux-4.14.12/mm/slab.c#3389*/3377  Static__always_inlinevoid*3378Slab_alloc (structKmem_cache *cachep, gfp_t flags, unsignedLongcaller)3379  {3380UnsignedLongsave_flags;3381    void*OBJP;3389Local_irq_save (save_flags);3390OBJP =__do_cache_alloc (Cachep, flags);3391Local_irq_restore (save_flags);3399    returnOBJP;3400}

In L3380 and L3389, if Local_irq_save () is a function, there must be a bug, because Save_flags's variable address needs to be passed to Local_irq_save ().

3380      Long save_flags; 3389      Local_irq_save (Save_flags);

However,Local_irq_save () and Local_irq_restore () are not functions, but macros , so there is no bug.

1. Implementation of Local_irq_save () and Local_irq_restore ()

/*linux-4.14.12/include/linux/irqflags.h#139*/ the#ifdef config_trace_irqflags ... the  #defineLocal_irq_save (Flags)111      Do {                                             theRaw_local_irq_save (Flags); 113Trace_hardirqs_off ();  the} while(0) the the117  #defineLocal_irq_restore (Flags)118      Do {                                            119             if(Raw_irqs_disabled_flags (Flags)) { -Raw_local_irq_restore (Flags); 121Trace_hardirqs_off (); 122}Else {                                123trace_hardirqs_on (); 124Raw_local_irq_restore (Flags);  the             }                                       126} while(0)...135  #else/* ! Config_trace_irqflags * *...139  #defineLocal_irq_save (Flags) $      Do {                                                    141Raw_local_irq_save (Flags); 142} while(0)143  #defineLocal_irq_restore (Flags) do {Raw_local_irq_restore (flags),} while (0)...146  #endif/* Config_trace_irqflags */

For the sake of simplicity, we are only concerned! The Config_trace_irqflags branch is just fine,

139  #define Local_irq_save (Flags)                                                                                        , 141              Raw_local_irq_save (flags);                       142      while (0) 143  #define Local_irq_restore (Flags) do {Raw_local_irq_restore (flags),} while (0)

Thus, we can assume that Locale_irq_save ()/local_irq_restore () is equivalent to:

#define Local_irq_save (Flags)    Raw_local_irq_save (flags)#define Local_irq_restore (Flags) RAW _local_irq_restore (Flags)

2. Implementation of Raw_local_irq_save () and Raw_local_irq_restore ()

/*linux-4.14.12/include/linux/irqflags.h#78*/ +  #defineRaw_local_irq_save (Flags) -       Do {                                             theTypecheck (unsignedLong, flags); BayiFlags =Arch_local_irq_save ();  the} while(0) the  #defineRaw_local_irq_restore (Flags) -       Do {                                             -Typecheck (unsignedLong, flags);  theArch_local_irq_restore (Flags);  the} while(0)

About Macro Typecheck () does not explain, because it is very intuitive, is to ensure that the type of flags must be unsigned long. Thus, Raw_local_irq_save () and Raw_local_irq_restore () are equivalent to:

#define Raw_local_irq_save (flags)       flags = Arch_local_irq_save ()#define Raw_local_irq_restore (flags)    Arch_local_irq_restore (Flags)

The following is an example of the implementation of the functions of Arch_local_irq_save () and Arch_local_irq_restore () x86.

3. Arch_local_irq_save () and Arch_local_irq_restore () the implementation of these two functions on the x86

/*linux-4.14.12/arch/x86/include/asm/irqflags.h#70*/ -  StaticInline Notrace unsignedLongArch_local_save_flags (void) in  { the      returnNative_save_fl (); the  } About the  StaticInline NotracevoidArch_local_irq_restore (unsignedLongflags) the  { theNATIVE_RESTORE_FL (flags); +}

Clearly, it's just a matter of understanding the specific implementation of NATIVE_SAVE_FL () and NATIVE_RESTORE_FL ().

Implementation of 3.1 NATIVE_SAVE_FL ()

/*linux-4.14.12/arch/x86/include/asm/irqflags.h#16*/ -  Staticinline unsignedLongNATIVE_SAVE_FL (void) -  { -UnsignedLongflags; + -      /** "=RM" is safe here, because "pops" adjusts the stack Before22 * It evaluates its effective address-- This was part of THE23 * documented behavior of the "Pop" instruction.24*/ -Asmvolatile("# __raw_save_flags\n\t" -                   "PUSHF; pop%0" -:"=rm"(Flags) -:/*No input*/ in:"Memory"); - to      returnflags; +}

This is an inline assembly code, followed by a simple demo to explain.

Implementation of 3.2 NATIVE_RESTORE_FL ()

。。。 Not to be continued ....

Understanding the Linux kernel for interrupt saving and recovery