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NAME="ROUTINES-MODULE-USE-COUNTERS"
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><TT
CLASS="FUNCTION"
>MOD_INC_USE_COUNT</TT
>/<TT
CLASS="FUNCTION"
>MOD_DEC_USE_COUNT</TT
>
<TT
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>include/linux/module.h</TT
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><P
> These manipulate the module usage count, to protect against
removal (a module also can't be removed if another module uses
one of its exported symbols: see below). Every reference to
the module from user context should be reflected by this
counter (e.g. for every data structure or socket) before the
function sleeps. To quote Tim Waugh:
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>/* THIS IS BAD */
foo_open (...)
{
stuff..
if (fail)
return -EBUSY;
sleep.. (might get unloaded here)
stuff..
MOD_INC_USE_COUNT;
return 0;
}
/* THIS IS GOOD /
foo_open (...)
{
MOD_INC_USE_COUNT;
stuff..
if (fail) {
MOD_DEC_USE_COUNT;
return -EBUSY;
}
sleep.. (safe now)
stuff..
return 0;
}
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> You can often avoid having to deal with these problems by using the
<TT
CLASS="STRUCTFIELD"
><I
>owner</I
></TT
> field of the
<SPAN
CLASS="STRUCTNAME"
>file_operations</SPAN
> structure. Set this field
as the macro <SPAN
CLASS="SYMBOL"
>THIS_MODULE</SPAN
>.
</P
><P
> For more complicated module unload locking requirements, you can set the
<TT
CLASS="STRUCTFIELD"
><I
>can_unload</I
></TT
> function pointer to your own routine,
which should return <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> if the module is
unloadable, or <SPAN
CLASS="RETURNVALUE"
>-EBUSY</SPAN
> otherwise.
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